three-term multiplication involving conversions

[jishnub]

These often show up in operations in normalized polynomial spaces, so it's worth improving the type-inference here.

julia> @inferred Multiplication(Fun(), NormalizedChebyshev())
MultiplicationWrapper : NormalizedChebyshev() → NormalizedChebyshev()
 0.0       0.707107   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   ⋅
 0.707107  0.0       0.5   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   ⋅
  ⋅        0.5       0.0  0.5   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   ⋅
  ⋅         ⋅        0.5  0.0  0.5   ⋅    ⋅    ⋅    ⋅    ⋅   ⋅
  ⋅         ⋅         ⋅   0.5  0.0  0.5   ⋅    ⋅    ⋅    ⋅   ⋅
  ⋅         ⋅         ⋅    ⋅   0.5  0.0  0.5   ⋅    ⋅    ⋅   ⋅
  ⋅         ⋅         ⋅    ⋅    ⋅   0.5  0.0  0.5   ⋅    ⋅   ⋅
  ⋅         ⋅         ⋅    ⋅    ⋅    ⋅   0.5  0.0  0.5   ⋅   ⋅
  ⋅         ⋅         ⋅    ⋅    ⋅    ⋅    ⋅   0.5  0.0  0.5  ⋅
  ⋅         ⋅         ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   0.5  0.0  ⋱
  ⋅         ⋅         ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋱   ⋱

[codecov]

Codecov Report

Patch coverage: 93.51% and project coverage change: -0.79 ⚠️

Comparison is base (2f401fa) 71.22% compared to head (142459e) 70.44%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #480      +/-   ##
==========================================
- Coverage   71.22%   70.44%   -0.79%     
==========================================
  Files          80       81       +1     
  Lines        8331     8466     +135     
==========================================
+ Hits         5934     5964      +30     
- Misses       2397     2502     +105     

Impacted Files	Coverage Δ
src/ApproxFunBase.jl	84.00% <ø> (+0.66%)	⬆️
src/LinearAlgebra/blas.jl	77.77% <ø> (ø)
src/Spaces/Spaces.jl	63.52% <ø> (-3.53%)	⬇️
src/Spaces/SumSpace.jl	70.44% <33.33%> (+0.77%)	⬆️
src/Operators/banded/CalculusOperator.jl	75.40% <80.64%> (+0.84%)	⬆️
src/Operators/functionals/CalculusFunctional.jl	79.48% <86.66%> (+7.07%)	⬆️
src/eigen.jl	96.42% <96.42%> (ø)
src/show.jl	95.68% <97.56%> (+12.85%)	⬆️
src/Operators/Operator.jl	70.70% <100.00%> (+3.13%)	⬆️
src/Operators/banded/ConstantOperator.jl	80.00% <100.00%> (+1.68%)	⬆️
... and 3 more

... and 47 files with indirect coverage changes

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